1. Field of the Invention
The present invention relates to a media separator to separate a recording medium for a fixing member by compressed air injection, a fixing device, and an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses including a fixing device such as a fixing roller to fix with heat and pressure an unfixed toner image onto a recording medium such as a sheet of paper have conventionally been widely known. Such fixing device is heated by a heat source such as a halogen heater. A pressure member such as a pressure roller is provided to press the fixing member. The sheet carrying the unfixed toner image thereon passes through a nip formed between the fixing member and the pressure member, and the toner image is fixed onto the sheet with heat and pressure.
A belt fixing method using a fixing belt as a fixing member is also well known. The fixing belt has a low thermal capacity and therefore can reduce warm-up time, and is energy efficient as well.
In the thermal fixing device as described above, because the toner image fused onto the sheet may contact the fixing roller or the fixing belt, the fixing roller or the fixing belt are coated with fluorine resins having excellent releasability. In addition, a separation claw is used to separate the sheet from the fixing member. A drawback of the separation claw, however, is a tendency to scrape a surface of the roller or the belt due to direct contact with the roller or the belt, which may produce streaks in an output image.
In general, in monochrome image printing, the fixing roller is configured to use a metal roller coated with TEFLON® (polytetrafluoroethylene, PTFE), with the result that its surface is strong enough to withstand scrapes due to the direct contact of the separation claw and its lifetime is long.
However, in color image formation, the fixing roller formed of a silicon rubber surface layer coated with fluorine resins (which is in general a Perfluoroalkoxy (PFA) tube several tens of microns thick) or a silicon rubber with a surface coated with oil is used to improve coloring effects. Such a roller has a soft surface layer which is apt to be damaged. If the surface layer is damaged, a streak-like flaw is generated on the fixed image. Accordingly, a contact means such as a separation claw is no longer used in the color image forming apparatus and instead a non-contact sheet separation has become the industry standard.
In the non-contact sheet separation configuration, if the adhesion between the toner and the fixing member is high, the sheet after fixation tends to wind around the roller of the belt, whereby a paper jam occurs easily. In color image formation in particular, because several toner layers are superimposed on the sheet one after another, the adhesiveness increases, thereby causing a paper jam to occur easily.
Accordingly, as a non-contact separation means, it is proposed to inject air to sheet separation position and is used.
JP-S61-59468-A discloses a fixing device to separate a sheet by injecting compressed air to a position between the sheet and the fixing roller even in a case of a sheet with a lower basis weight.
The air separation mechanism in which a recording medium is separated from the roller by a force of the compressed air as disclosed in JP-S61-59468-A includes a nozzle to inject compressed air. The compressed air is directed toward the recording medium to be conveyed from the nip portion outlet between the fixing roller and the pressure roller to separate the recording medium from the roller.
However, as illustrated in FIG. 16, when the basis weight of the sheet is large, the amount of toner carried on the edge portion of the sheet and on the entire sheet is small, and the sheet does not absorb moisture, the sheet does not wind around the fixing roller, is separated therefrom, and is conveyed from the nip portion. By contrast, if the sheet basis weight is large, an abundant amount of toner is carried on the sheet, and the sheet absorbs moisture, that is, in a state in which the sheet tends to wind around the fixing roller, the sheet is separated and conveyed while winding around the fixing roller from the nip end portion as illustrated in FIG. 17, whereby an excessive amount of heat beyond that which is necessary is applied to the toner and defective images are formed.
To separate and convey the sheet without the sheet wrapping around the roller, the pressure and the flow amount of the injected compressed air need to be increased. However, excessive pressure of the compressed air destabilizes moving of the type of the sheet with a large basis weight, less unfixed toner amount on a leading edge and on an entire surface, and without moisture absorbed, thereby causing defective conveyance of the sheet.